This invention relates to couplings for drivingly connecting power transmission shafts.
There are many industrial applications where it is required to connect the ends of two power transmission shafts together in such a manner that, whilst torsional and radial rigidity is maintained, the connecting coupling or device allows the shafts to move relative to one another along their common axis. A typical source of such relative movement might be where the two shafts are respective output and input shafts of a prime mover and a piece of driven machinery, for example turbine and compressor. Depending on the relative dispositions of foundation attachment points, locating thrust bearings and other factors of say, the turbine and compressor, the normal temperature changes in operation of such an assembly of machinery will produce thermal expansion or contraction of the distance between the two opposed shaft ends.
Usually the coupling for such an installation will also incorporate means for allowing angular and lateral relative displacement of the shaft centre lines, for example two hooks joints separated by a cardan shaft, but often as with the hooks joint there is either nil or insufficient freedom for relative axial displacement. The present invention is concerned with such freedom for purely axial displacement.
The invention has several important advantages over some of the existing devices which already perform the function described above. For instance, it does not involve frictional sliding motion, requires no lubrication and retains a high degree of rigidity throughout its working life against torsional, angular and lateral (radial) relative movement of the shaft connection.
In order to understand what these advantages mean in terms of coupling operation, it is best first to examine a typical form of axially sliding shaft coupling for example a splined joint, and show some of its disadvantages.
In the splined joint, torsional drive is transmitted through mating male and female splines, which allow freedom of axial movement within certain limits as dictated by the particular design of coupling. Because the contact pressures on the flanks of the splines are usually fairly high, it is necessary to introduce some form of lubrication, not only to retard wear, but also to minimise sliding frictional forces. Otherwise, excessive axial loads could be applied to the shaft supporting bearings when relative axial movement was imposed. The introduction and retention of such lubrication, particulary in high speed applications, can be extremely complex and costly.
Some degree of clearance between male and female components is always required, both in the interest of manufacturing tolerances and to allow ingress of lubricant. It is inherent in the design therefore, that torsional backlash and some lack of radial and angular rigidity will inevitably be present, even with the coupling in new condition.
When wearing of the splines takes place in service, two important effects are produced. The first is increased backlash, radial and angular, freedoms of movement, which if the coupling is on a high speed application, will cause increased out-of-balance forces to be generated i.e. vibration. The second and possibly more damaging is that spline wear may occur when the coupling runs for long periods in one axial position, such that a step is formed in each of the splines. If the coupling is then required to accommodate shaft movement which means overriding this step with torque applied then very high axial loads may be produced, with consequent damage to the shaft bearings.
The coupling of the present invention obviates or mitigates both these disadvantages and requires no lubrication.